1. Field of the Invention
The present invention generally relates to a light-emitting composition, and more particularly, to a phosphor having high brightness and a luminescent device.
2. Description of Related Art
Light emitting diodes are extensively used in semiconductor-based luminescent devices at present, such luminescent devices are provided with the advantages of high luminescence efficiency, small size, low power consumption and low cost, thus they are applicable for use in various types of light sources. Semiconductor-based luminescent devices comprise a semiconductor light-emitting element and a phosphor, in which the phosphor may absorb and convert the light emitted from the semiconductor light-emitting element. The light emitted from the semiconductor light-emitting element and the light converted and emitted from the phosphor are mixed and utilized. Such luminescent devices are applicable for use in various areas, including fluorescent lamps, vehicles lighting, monitors, backlit liquid crystal displays, and the like. In which, white luminescent devices are the most extensively used.
Current white luminescent devices are assembled by means of a YAG (yttrium aluminum garnet) phosphor (Y3Al5O12:Ce) with cerium as the active center and a semiconductor light-emitting element emitting blue light. However, using the mixed light emitted from a Y3Al5O12:Ce phosphor combined with a semiconductor light-emitting element emitting blue light, the color coordinates of the mixed light are positioned on the connecting line between the color coordinates of the blue light emitted from semiconductor light-emitting element and the color coordinates of the light emitted from Y3Al5O12:Ce phosphor. Hence, the emitted mixed light is white light deficient in red light, and color rendering properties and color saturation are clearly insufficient. In addition, the preferred region of the excitation spectrum of the Y3Al5O12:Ce and the luminescence region of the semiconductor light-emitting element are inconsistent, thereby causing poor conversion efficiency of excitation light, and a high brightness of white light source is difficult to obtain. In order to resolve such phenomena, YAG:Ce phosphors mixed with phosphors emitting red light have been developed in recent years, and such phosphors include Sr2Si5N8:Eu phosphors with europium (Eu) as the active center, CaAlSiN3:Eu phosphors and sialon phosphors are known. However, because the crystal itself has poor heat resisting properties, thus, the Sr2Si5N8:Eu phosphor has the disadvantages of decrease in brightness and color rendering properties after long term usage. And, although the sialon phosphor itself has no durability problems, however, luminescence brightness of the phosphor is clearly insufficient, and thus not commercially popular. Although CaAlSiN3:Eu phosphors have preferred durability, and provide better brightness compared to sialon phosphors, however, industries are still expecting further improvement in the luminescence brightness of the phosphor, thereby enabling the luminescent device to be provided with higher luminescence efficiency.